Styrene diene living block polymer

Figure 3. Chemical modification of styrene-diene living block polymer.

In addition to the triblock thermoplastic elastomers, other useful copolymers of styrene with a diene are produced commerically by living anionic polymerization. These include di-and multiblock copolymers, random copolymers, and tapered block copolymers. A tapered (gradient) copolymer has a variation in composition along the polymer chain. For example, S-S/D-D is a tapered block polymer that tapers from a polystyrene block to a styrene-diene random copolymer to polydiene block. (Tapered polymers need not have pure blocks at their ends. One can have a continuously tapered composition from styrene to diene by... 

So far the discussion was focused on copolymers derived from a mixture of styrene and a diene. In view of the "living" nature of organolithium polymerization, it is also possible to synthesize block polymers in which the sequence and length of the blocks are controlled by incremental (or sequential) addition of monomersr This general method of preparing block polymers is readily adaptable to commercial production, and, indeed, a number of block copolymers are manufactured this way. Those that have received the most attention in recent years are the diene-styrene two-phase... 

Preparation of styrene-diene block copolymers involves sequential addition of the diene monomer to "living" polystyrenes. The resulting polymer is an AB block copolymer consisting of segments of styrene mers and diene mers. 

First, there is a diene-rich block a middle block follows which is initially richer in butadiene with a gradual change in composition until eventually it becomes richer in styrene a final block of styrene completes the stmcture. Thus, there is compositional homogeneity between polymer chains, but there is compositional heterogeneity within each polymer chain because of the living nature of these polymerizations and the disparity in the monomer reactivity ratios. 

Difunctional initiators such as sodium naphthalene are useful for producing ABA, BABAB, CAB AC, and other symmetric block copolymers more efficiently by using fewer cycles of monomer additions. Difunctional initiators can also be prepared by reacting a diene such as /n-diisoprope ny I benzene or l,3-bis(l-phenylethenyl)benzene with 2 equiv of butyl-lithium. Monomer B is polymerized by a difunctional initiator followed by monomer A. A polymerizes at both ends of the B block to form an ABA triblock. BABAB or CABAC block copolymers are syntehsized by the addition of monomer B or C to the ABA living polymer. The use of a difunctional initiator is the only way to synthesize a MMA-styrene-MMA triblock polymer since MMA carbanion does not initiate styrene polymerization (except by using a coupling reaction—Sec. 5-4c). 

Note that styrene and conjugated dienes can be copolymerized to yield statistical or block copolymers. The latter process, which involves additions of one monomer to a living polymer of the other monomer, is described in the following section. 

The NIR in situ process also allowed for the determination of intermediate sequence distribution in styrene/isoprene copolymers, poly(diene) stereochemistry quantification, and identification of complete monomer conversion. The classic one-step, anionic, tapered block copolymerization of isoprene and styrene in hydrocarbon solvents is shown in Figure 4. The ultimate sequence distribution is defined using four rate constants involving the two monomers. NIR was successfully utilized to monitor monomer conversion during conventional, anionic solution polymerization. The conversion of the vinyl protons in the monomer to methylene protons in the polymer was easily monitored under conventional (10-20% solids) solution polymerization conditions. Despite the presence of the NIR probe, the living nature of the polymerizations was maintained in... 

Sequential polymerization with site transformation Block copolymers in which the active sites are not compatible are usually prepared in two steps. First, a site transformation is performed, after which initiation of the ROP occurs. Block copolymers with a poly(styrene) or a poly(butadiene) block and a poly(carbonate) block, for instance, are obtained after site transformation of a carbanionic to an alcoholate species [25]. The initiation reaction of the ROP of DTC using living vinyl (or diene) polymers with alkali metal counterions as initiators, was performed after site transformation of the carbanionic into the alcoholate species... 

---